1. Field of the Invention
The present invention relates to an organic electroluminescence element.
2. Description of the Background Art
An organic electroluminescence element (hereinafter referred to as an organic EL element) is expected as a new self-light emission type element. The organic EL element has such a stacked structure that a carrier transport layer (an electron transport layer or a hole transport layer) and a luminescent layer are formed between a hole injection electrode and an electron injection electrode.
An electrode material which has a large work function such as gold or ITO (indium-tin oxides) is employed for the hole injection electrode, while a material which has a small work function such as Mg (magnesium) or Li (lithium) is employed for the electron injection electrode.
Organic materials are employed for a hole transport layer, a luminescent layer and an electron transport layer. Materials which have the property of p-type semiconductors are used for the hole transport layer, while those which have the property of n-type semiconductors are used for the electron transport layer. The luminescent layer also has carrier transportability such as electron transportability or hole transportability and is composed of an organic material, which emits fluorescence or phosphorescence.
The hole injection electrode, the hole transport layer, the luminescent layer, the electron transport layer and the electron injection electrode are stacked in this order so as to form an organic EL element.
Such function layers as the hole transport layer, the electron transport layer and the luminescent layer are formed of a plurality of layers, or alternatively, some of those layers are deleted depending on the organic materials to be employed.
In such an element structure as shown by Chihaya Adachi et al. in Appl. Phys. Lett., Vol. 55, pp. 1489–1491(1989), for example, there exists only two organic layers of a luminescent layer and an electron transport layer between a hole injection electrode and an electron injection electrode. This is because the luminescent layer also serves as a hole transport layer since the luminescent layer made of NSD as a luminescent material has excellent hole transportability.
In such an element structure as shown by C. W. Tang et al. in Appl. Phys. Lett., Vol. 51, pp. 913–915(1987), the structure is formed of two organic layers of a hole transport layer and a luminescent layer. In this case, Tris(8-hydroxyquinolinato)aluminum (hereinafter referred to as Alq) of the luminescent layer serves to both emit light and transport electrons.
On the other hand, such an element structure as shown by S. A. Van Slyke et al. in Appl. Phys. Lett., Vol. 69, pp. 2160–2162(1996) is formed of three organic layers of a hole injection layer, a hole transport layer and a luminescent layer. In this case, the hole injection layer is composed of copper phthalocyanine and serves the same as the hole transport layer. That is, there exists two hole transport layers in the whole element.
Thus, free adjustments can be made in the number of layers constituted by the electron transport layer, hole transport layer and luminescent layer in dependence on the organic materials to be employed.
While the organic EL element is formed of the respective function layers of the electron transport layer, hole transport layer and luminescent layer, the organic materials now in use have insufficient transportability of electrons or holes. This provides such problems as low luminance or low luminous efficiency.
In particular, the electron transport layer has become the cause of such problems. As a material which has been widely used, Alq as described above is employed as an electron transport material. This organic material can also be employed as a luminescent material and can easily form films by vacuum vapor deposition. Further, this material has such excellent characteristics as high stability after film formation and high heat resistivity. However, the material provides poor carrier transportability of holes and electrons, exhibiting poor luminescent characteristics.
The disadvantage of Alq is that Alq deteriorates while being energized. (Refer to Hany Aziz et al., Science, Vol. 283, pp. 1900–1902(1999).) This results from deterioration of a molecule itself, which occurs if holes flow into Alq.
In Appl. Phys. Lett., Vol. 75, pp. 172–174(1999), Vi-En Choong et al. have proposed employing N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (hereinafter referred to as NPB), which is a hole transportable material, for Alq in order to solve the above disadvantage of Alq; however, such an approach has not yet overcome the above described low luminous efficiency.